Dielectric heating apparatus



Sept. 29, 1959 w. RUEGGEBERG DIELECTRIC HEATING APPARATUS Filed July 9, 1956 INVENTOR WERNER RUEC-GEBERG ATTORNEY United States Patent Ofiice 2,905,961 Patented Sept. 29,1959

DIELECTRIC HEATING APPARATUS Werner Rueggeberg, East Hempfield Township, Lancasvter County, Pa., assignor to Armstrong Cork Company, Lancaster, Pan, a corporation of Pennsylvania Application July 9, 1956, Serial No. 596,491

8 Claims. (Cl. 331-177) This invention relates generally to an electronic power generator and more particularly to a resonant line power generator for dielectric heating.

Dielectric heating is often useful in removing water from various materials. Such heating may be accomplished rapidly with the further advantage that the heat is generated uniformly throughout the volume of the material being dried.

One of the drawbacks of drying with dielectric heating, however, is that the dielectric constant of the material to be dried usually changes as the drying progresses. These changes may cause frequency variations in the generator which in turn adversely affect the power level once the frequency in the system deviates from that for which the system was designed. This condition is particularly evident with high Q (low power factor) loads. Hence most dielectric heating equipment suitable for drying low power factor loads has had to operate in narrow frequency ranges or suffer drastic power losses.

The primary object of the present invention is to present dielectric heating apparatus for the drying of materials which will maintain a high power delivery over a wide range of frequencies around a center frequency. A further object is to present dielectric heating apparatus having unusual power insensitivity to frequency variations where wide ranges of load capacitive reactance are encountered. Other objects and advantages of the invention and the preferred embodiment thereof will be evident from the following description.

The invention contemplates a resonant line power generator comprising a coaxial line having an electrical length in the range of 49 to 85 (dependent on instantaueous frequency) and having substantially infinite parallel reactive impedance between the inner and outer conductor'thereof at the input terminals. The load is connected across the inner and outer conductor at the output terminals. A terminating inductance is connected between the load and one of the inner and outer conductors. The size of this inductance in henrys must be of such magnitude to satisfy the equation (w. LC 1) (1) on wherein Z =characteristic impedance of said coaxial line, ohms w=angular frequency of operation, radians per second L=terminating inductance, henrys C =-load capacity, farads p=phase constant of line, radians per inch l=physical length of coaxial line, inches A blocking capacitor is placed on the input side of the coaxial line to eliminate ,direct current voltage thereon. And finally, there is a means for supplying radio frequencyenergy to said inner conductor and said outer conductor through electronic power tube circuitry.

- In the drawing, the single figure illustrates a two-tube, schematic, simplified assembly of components of a dielectan Bl= 2 tric heater of the present invention. The apparatus can be made with one' tube and one coaxial line by utilizing either the upper half or lower half of the apparatus depicted in the drawing.

A conventional vacuum tube triode 1 contains a plate 2, a grid 3, and a cathode 4, which cathode may be directly or indirectly heated. The plate 2 is connected to a blocking condenser 5, which in turn is connected to the inner conductor 6 on the input side of a coaxial line; The output side of the inner conductor 6 is connected to a terminating inductance 7, which is then connected to the load electrodes 8 and 9. The outer conductor 10 of the coaxial line is grounded as at 11. The direct current input to the entire circuit is connected between a ground and +13 at the center of inductance 12 So far as radio frequency voltages are concerned, the ground 13 is connected through a grid bypass and blocking capacitor 14 to the variable grid inductance 15 and the grid capacitance 16 (by virtue of the circuit) and thence to grid 3. The grid 3 is maintained at its proper negative bias voltage by virtue of the direct component of current that fiows over the bias resistance 17. The grid-plate capacitance 18 generally exists across the plate 2 and the grid 3 through internal interelectrode capacitive coupling of the vacuum tube.

In the drawing the plate 2 is connected to the inner conductor 6 and thence to the load electrodes 8 and 9. This is the preferable embodiment, although it must be pointed out that the plate 2 could be connected to the outer conductor 10, which would then be connected to the load electrodes -8 and 9; the inner conductor 6 would then be grounded and connected as is the outer conductor 10 in the drawing. However, this latter modification is not desirable since it places the high voltage on the outer conductor 10 and thus impressesan unnecessary hazard to personnel on the apparatus.

The apparatus of the present invention can be made to deliver power that is proportional to the series resistive component of the work load impedance regardless of its dielectric constant. Stated another way, when the ap paratus is used to remove water, the nature of the particular material to be dried is such that the series resistance of the load is very nearly proportional to the water content of the material to be dried over a surprisingly wide range of water content. Consequently, the power level maintains water content proportionately for variations in dielectric constant of the material to be dried from plus 1 to very high values. In certain other applications, moreover, the dielectric constant may assume infinitely large values, depending on the nature of the material being heat-treated, while the generator maintains its steady power delivery properties.

In prior quarterwave lines, a change of dielectric constant in the work load will cause power changes mostly as a result of frequency changes, since the current remains essentially constant around'the configuration. But in the partial wave line of the present invention, the current varies to compensate for frequency changes in such a manner that the power is maintained substantially constant.

'In the design of'a dielectric heating apparatus according to the present invention, there will be involved a compromise choice between'frequency, length of line, and'characteristic impedance of the line. The first two are somewhat interdependent and will depend on the physical length of line possible, electrode dimensions, and frequency preferences. The second parameter, characteristic impedance, is chosen primarily in order to realize the load current required for a desired power level at the heating electrodes.

Calculations have proved that 67 is the optimum electrical length of the line for minimum rate of change in 3 power with change of dielectric constant. So in each case the apparatus of the present invention will be designed for an electrical length of 67, recognizing that the electrical length may then wander between the 49- 85 limits necessary to maintain power changes within 210% limits.

In designing a particular dielectric heater according to the present invention, the power, P required for the particular purpose will be known or can be readily calculated. The series resistance, R of the load will also be known. Thus the effective load current, 1 will be determined by the equation The effective radio frequency voltage, E,,, at the input of the coaxial line will be determined by the voltage of the direct current power supply to the tube, and hence is With all the above information, the required inductive reactance. X needed at the output of the coaxial line can be calculated from the equation After solving Equation for X the necessary terminating inductance, -L, is determined by the equation in 21rf Thus, the design of a dielectric heater according to the present invention may be readily carried out by those skilled in the art. In all cases, the terms of definitive Equation 1, given earlier, will be met. It will be appreciated that Equation 1 defines any apparatus of the present invention once built, while Equations 3 and 5 serve as tools for bui ding an apparatus of the present invention to serve a particular need.

The products to be treated with the present apparatus may vary widely. To illustrate, however, the description will be limited to a product whose thickness is small compared to at least one other dimension. In particular, the description will be confined to the drying of an acoustical board whose thickness is small compared to its length and width and whose composition may generally be stated to be mineral wool and a binder system. Generally speaking, these boards measure about 1" to 2" in thickness and about 4 to 5 in length and width. Such boards well illustrate a problem commonly encountered in drying in that the last l%5% of water to be removed is the most difiicult. This is true since conventional oven equipment requires the transferring of heat through the outer dry sections to the wet inner section of the board. Since the board is somewhat sensitive to heat degradation, the problem may not be resolved by simply subjecting it to higher temperatures.

The die'ectric heating unit of the present invention is preferably arranged in the total drying system to replace the last sections of a conventional oven, or it may be arranged as a unit in a series of individual dielectric heating units, which series may replace the last sections of a conventional oven. This latter embodiment is illustrated in copending application Serial No. 560,943, filed January 24, 1956, by George E. Gard. The present apparatus serves unusually well as the third unit in the Gard system of dielectric heating.

With these considerations in mind, a dielectric heating apparatus was designed to remove water from the boards described above. The physical space available for the apparatus was too small to accommodate the optimum 67 length at the necessary frequency. Accordingly, a 58.5 line was used with completely satisfactory results. The frequency variations which are caused by the varying dielectric constant of the boards containing 0 to 5% moisture corresponded to a variation of electrical length of line from 61 to 58. This change for a given load series resistance affected the power level by approximately plus or minus 2% around the center frequency of 23.65 megacycles per second. Moreover, when the dielectric constant variation assumed the complete limiting values of from plus 1 to plus infinity, the resultant frequency variation caused a line length shift from 61 to 54 which corresponded to a change in power level of approximately plus or minus 2.5% around the 57.5 value. Where the optimum 67 electrical length can be used, even better results will be obtained. So long as the electrical length varies between the limits of 49 and the excellent results achieved as described herein may be accomplished. The 49 and 85 lengths represent, as mentioned earlier, a power departure limit of :10% of optimum power at 67".

In the present invention, a partial resonant line, inductively terminated, serves as the tank circuit in a conventional tuned plate-tuned grid oscillator circuit. The result of such a system is that the power proportioned match is essentially maintained over a range of frequencies extending to plus or minus 27% of the center frequency. As mentioned earlier, the unusual power insensitivity of the system to frequency variations makes it particularly useful in many dielectric heating applications where wide ranges of load capacitive reactance are encountered. The system is particularly well suited to the heating of high Q (low power factor) loads, because under this type of loading a low impedance line is mandatory. Such a line, moreover, represents an equivalent high energy content tank circuit which will mask the effects of grid circuit impedance. Under thiscombination of conditions, then, the oscillator will assume very closely the constant power properties (within i10%), in spite of frequency variations, that may be realized through use of a nominal 67 resonant line resonator.

I claim:

1. A resonant line power generator for dielectric heating comprising in combination, a coaxial line having an electrical length adapted to wander in the range of 49 to 85 as the frequency changes and having substantially infinite parallel reactive impedance between the inner and outer conductors of said coaxial line at the input end, a load connected across said inner and outer conductor, a terminating inductance connected between said load and one of said inner and outer conductors, said inductance being of a magnitude to satisfy the equation o (w LCT:1 tan 31:0

wherein a blocking capacitor on the input side of said coaxial line to eliminate direct current voltage thereon, and means forsupplying radio frequency energy to said one of said inner and outer conductors.

2. A resonant line power generator according to claim 1 having a single coaxial line.

3. A resonant line power generator according to claim 1 having two coaxial lines mutually coupled at the output and wherein said means for supplying radio frequency energy comprises a push-pull generator.

4. A resonant line power generator according to claim 1 wherein said nominal electrical length of said coaxial line is 67.

5. A resonant line power generator according to claim 1 wherein said nominal electric length of said coaxial line is 58.5".

6. A resonant line power generator according to claim 1 wherein said terminating inductance is connected between said inner conductor and said load.

References Cited in the file of this patent UNITED STATES PATENTS 2,229,205 Braaten Jan. 21, 1941 2,414,991 Wheeler Jan. 28, 1947 2,724,055 Bliss Nov. 15, 1955 2,740,889

Eckert Apr. 3, 1956 

